This invention relates to a blower proving system for a fuel burner and, more specifically, to such a system utilizing a fan rotational speed sensor.
For many fuel burner installations, it is desirable to provide a purge period prior to the initiation of fuel flow and the occurrence of ignition at a burner. During the purge period, a blower is energized to dissipate any fuel vapor that may have accumulated in the combustion chamber because of some inadvertence, such as a leaky fuel valve. The dissipation of such residual fuel vapor reduces the possibility of an explosion during an attempt to ignite newly released fuel. However, the safety provided by a purge period can be comprised by a malfunctioning blower system that fails to adequately dissipate residual fuel vapor in the vicinity of the vapor. Such a malfunction can result from either a mechanical failure of the blower itself or some type of obstruction that prevents the blower from forcing air through a purge zone occupied by the burner. To eliminate the danger associated with defective blower systems, some fuel burner arrangements employ air-proving devices that monitor the presence of purging air near the burner prior to an attempt at ignition. Generally, the air-proving devices use detectors that sense either air flow or air pressure in the vicinity of the burner. The most common type of detector is a pivotally mounted flat plate that is moved by air velocity to operate a microswitch. Closing the microswitch establishes a ready signal that is required for ignition. Such detectors are commonly referred to as sail switches. A serious deficiency of sail switches is their tendency to stick in the "on" position resulting in the presence of a ready signal in even those instances in which a blower system has malfunctioned. Furthermore, no provision exists for indicating such a stuck switch condition.
The object of this invention, therefore, is to provide a system used to provide purging air to fuel burner installations.